1. Field of the Invention
The present invention relates to a sheet conveying apparatus which conveys a sheet placed on a sheet feed tray to a predetermined position and an image reading apparatus which optically reads an image on a sheet conveyed by the sheet conveying apparatus and outputs read data.
2. Description of Related Art
Conventionally, in an original conveying apparatus mounted on an image reading apparatus such as a copying machine or printer, a sheet sometimes skews to cause a lateral shift when being conveyed. As the sheet causes a lateral sheet, an image formed on the sheet may tilt or shift. For this reason, in general, a lateral shift is detected by a sensor, and the shift of an image is corrected on the basis of the detection result.
In addition, when the lateral shift of a sheet is detected and the image reading position is corrected in accordance with the lateral shift amount, a light-receiving element having many pixels such as a line sensor or image sensor represented by a photodiode array, CCD sensor, CMOS sensor, or CIS sensor is generally used as a lateral shift amount detection sensor, and the lateral shift position of the sheet is detected in accordance with the ON/OFF state of each pixel (see, for example, Japanese Unexamined Patent Publication No. 6-92512: patent reference 1, Japanese Unexamined Patent Publication No. 7-187449: patent reference 2, and Japanese Unexamined Patent Publication No. 2002-292960: patent reference 3). In this case, the light-emitting side requires a light-emitting area similar to the length of the light-receiving side, and hence parallel light is formed by using an LED array, diffusion plate, lens, or slit.
A general image reading apparatus handles sheets of various sizes. As schemes of conveying such sheets of different sizes, an one-side reference convey scheme and center reference convey scheme have already been known. In the former scheme, sheets of all sizes are conveyed while one end of each sheet in the widthwise direction is aligned with a reference position set on one side of a sheet feed tray. In the latter scheme, sheets of all sizes are conveyed while the center of each sheet in the widthwise direction is aligned with the center of a sheet feed tray.
According to the one-side reference convey scheme, sheets of all sizes can be detected by placing a lateral shift amount detection sensor within a relatively short range at a position on a convey path which corresponds to the reference position provided on one side on the sheet feed tray. In contrast, according to the center reference convey scheme, since the convey positions of end portions of sheets of the respective sizes differ from each other, at least the light-receiving element of a lateral shift amount detection sensor needs to have a length corresponding to the positions of the end portions of the respective sheets in the widthwise direction. For this reason, the light-receiving element becomes long. On the other hand, the resolution of an output voltage from a current-voltage conversion circuit is uniform. As a consequence, the detection precision of the lateral shift amount of a sheet in this case is lower than that in a case wherein the light-receiving element is short (in the case of the lateral shift amount detection sensor in the one-side reference convey scheme). In addition, if almost uniform light does not strike the light-receiving element, the detection precision deteriorates. For this reason, as described above, a light-emitting area (parallel light) almost equal in length to the light-receiving element must be formed on the light-emitting side by using an LED array, diffusion plate, lens, or the like. Furthermore, as the light-receiving element becomes longer, a lager space is required, and an increase in cost occurs.